Williams said that preliminary testing of bags found at the scene of the overdoses and stamped with the word "Theraflu'' appear to show that they contain fentanyl, a synthetic morphine substitute that is roughly 100 times more powerful than morphine.

It resembles heroin, though it is much more potent, and it has been blamed for dozens of deaths around the U.S.

On Saturday, Williams' office said 14 people in Allegheny County had died from heroin overdoses in the past week. Deaths also have been reported in Westmoreland County, Pittsburgh Mayor Bill Peduto's office said.